. Outlines of zoology. gas in the bladder, due to the varying hydrostaticpressure. CLASSIFICATION OF FISHES Sub-Class I. Elasmobranchii. Cartilaginous Fishes, Sharks and Skates Voracious carnivorous fishes, with cartilaginous skeleton,placoid scales, usually heterocercal tails, claspers ono/,^_ the pelvic fins of the males. Except in Holo-cephali there is no coverover the (5-7) gill-aper-tures ; anterior to thesethere is often a spiracle—the first gill-cleft —with a. rudimentary gill - clefts areseparated by completesepta, and the gill-filaments are attachedthroughout their length
. Outlines of zoology. gas in the bladder, due to the varying hydrostaticpressure. CLASSIFICATION OF FISHES Sub-Class I. Elasmobranchii. Cartilaginous Fishes, Sharks and Skates Voracious carnivorous fishes, with cartilaginous skeleton,placoid scales, usually heterocercal tails, claspers ono/,^_ the pelvic fins of the males. Except in Holo-cephali there is no coverover the (5-7) gill-aper-tures ; anterior to thesethere is often a spiracle—the first gill-cleft —with a. rudimentary gill - clefts areseparated by completesepta, and the gill-filaments are attachedthroughout their lengthto the septa. The mouthextends transversely onthe under side of thehead. The nostrils arealso ventral. There isno air-bladder. A spiralfold extends along theinternal wall of the largeintestine. Into the ter-minal chamber (or clo-aca) of the gut the genitaland urinary ducts alsoopen. The ventricle ofthe heart has a con-tractile conus is ova are few andlarge, with much Fig. 301.—Young skate.—From Beard. The yolk-sac has been cut off, the yolk-stalk isleft, ^ Mouth ; nostril, e.^. * externalgills ; fl., clcica ; c, claspers. ELASMOBRANCHII. 567 Large egg-purses are common, but some Elasmobranchsare viviparous. The embryos have gill-filaments projectingout of the gill-clefts, so-called external gills. They arereally elongated internal gills. Elasmobranchs retain moreembryonic features, the naso-buccal groove and auditoryopening, than other fishes. Order i. Plagiostomi or Selachii With transverse ventral mouth, pre-oral rostrum, uniserial pairedfins, claspers, heterocercal tail, usually five pairs of open gill-clefts. Subdivisions.—(i) The older Selachoidei, with approximatelycylindrical bodies and lateral gill-openings, as in shark and dogfish;(2) the more modified Batoidei, with flattened bodies, ventral gill-openings, and pectoral fins joined to the head, as in skates or rays. Mustelus, Carcharias, Squalu
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Keywords: ., bookcentury1900, bookdecade1910, bookidcu3192, booksubjectzoology
